1. Technical Field
The embodiments herein generally relate to demodulation techniques in a receiver, and, more particularly, to digital downconversion and fast channel selection of narrowband signals using a wide band RF tuner in the receiver.
2. Description of the Related Art
A tuner is primarily designed to receive and down-convert the wireless and wire-line signals from radio frequency (RF) signal to the required intermediate frequency (IF). Typically, the tuner is designed based on the application and bandwidth of the signal that needs to be received. When a single tuner is used to receive signals with multiple bandwidths, it fails to meet selectivity and sensitivity requirements for signals with lower bandwidth.
For a signal with bandwidth equals to B1, a tuner is defined as wide band tuner if it is designed to receive a signal with bandwidth equals to at least 2*B1, and signal is defined as a narrowband signal. FIG. 1 is a table view illustrating narrowband standards having standards field 102, a bandwidth field 104, and sensitivity details of narrowband standards field 106. According to the table shown in the FIG. 1, this requires external components to meet required sensitivity if a wide band tuner is used to receive such signals. For an example FM radio with 200 KHz of bandwidth doesn't meet the required selectivity, if the received signal is demodulated in analog domain using an analog front-end designed for a TV standard.
Hence, a typical narrowband demodulator uses a dedicated analog front-end to select such a narrowband signal, which further requires multiple scan to detect all active band signals, by demodulating the selected band signals in digital or analog domain. Additionally, narrowband demodulation on a multi-standard platform that are also capable of demodulating a wide band Analog/Digital signals, requires additional analog components to meet selective and sensitivity requirements for individual standards, and thus increases the overall area, complexity, and the cost.
Most of the conventional narrowband receivers use a narrowband SAW (surface acoustic wave) filter, or analog band pass filters for required band signal selection. In such receivers, the overall performance of the receiver is highly influenced by analog filters that are used to reject adjacent band signals. Typically, such filters are made of crystal, and are limited to 10 dB rejection without forfeiting phase linearity. It is also required to use multiple stages for filtering, to achieve selectivity and sensitivity requirements of a receiver. This also increases the overall area, power consumption, complexity, and the cost.
Recent, state-of-art digital technology has made an attempt to demodulate analog standards in digital domain, which has simplified signal gain control and filtering required for adjacent channel rejection. Such digital filters are cheap to implement, and may have precisely linear phase, reliable compared to analog filters, and can be configured based upon performance requirements. Most of the existing digital receivers perform filtering in analog domain for an active band signal selection, and only demodulation is done in digital domain.
FIG. 2 illustrates a block diagram of a typical wide band receiver 200 according. The wide band receiver 200 includes a tuner 202, a filter 204, an analog to digital converter (ADC) 206, a digital signal processor (DSP) 208, and a digital to analog converter (DAC) 210. The tuner 202 receives a required narrowband signal and down converts into an intermediate frequency (IF) signal. The tuner 202 may select multiple narrowband signals at the same time. The filter 204 performs filtering in the analog domain before the IF signals are converted to digital signals.
The filter 204 rejects unwanted adjacent band signals from the IF signals. In the FIG. 2, an analog band pass filter or a surface acoustic wave (SAW) filter is used to reject the unwanted adjacent band signals. The ADC 206 converts the IF signal to a digital signal. The DSP 208 processes the signal in a digital domain for adjacent band signal rejections, sample rate conversions, and demodulation. For analog standards, the DAC 210 is used to convert demodulated digital signals to a required analog signal. The receiver 200 requires one or more band pass filters to perform filtering in the analog domain before the IF signals are converted to digital signals. Therefore, all narrowband signals are not detected at the output of the ADC 206 and the DSP 208, since the filtering is performed in the analog domain. Accordingly, there remains a need for a demodulation technique to achieve selective and sensitivity of signals in a receiver without using any external component.